Lakhmir Singh & Manjit Kaur: Gravitation, Solutions- 1 | Science Class 9 PDF Download

Page No - 100

Question 1:
What is the value of gravitational constant G (i) on the earth, and (ii) on the moon ?
Solution :
Value of gravitational constant G on the earth and the moon is = 6.67 x 10^-11 Nm²/kg²
Note that the value of G always remains constant irrespective of the location. 

Question 2:
Which force is responsible for the moon revolving round the earth ?
Solution :
Gravitational force is responsible for the moon revolving round the earth.

Question 3:
Does the acceleration produced in a freely falling body depend on the mass of the body ?
Solution :
No, the acceleration produced in a freely falling body is independent of the mass of the body.

Question 4:
Name the scientist who gave the three laws of planetary motion.
Solution :
Johannes Kepler gave the three laws of planetary motion.

Question 5:
Name the scientist who explained the motion of planets on the basis of gravitational force between the sun and planets.
Solution :
Newton explained the motion of planets on the basis of gravitational force between the sun and planets.

Question 6:
State the Kepler’s law which is represented by the relation r³ ∝ T².
Solution :
Kepler’s law of periods states that: The cube of the mean distance of a planet from the sun is directly proportional to the square of time it takes to move around the sun.

Question 7:
Which of the Kepler’s laws of planetary motion led Newton to establish the inverse-square rule for gravitational force between two bodies ?
Solution :
Kepler’s third law of planetary motion led Newton to establish the inverse-square rule for gravitational force between two bodies.

Question 8:
Name the property of earth which is responsible for extremely small acceleration being produced in it as a result of attraction by other small objects.
Solution :
Extremely large mass of the earth.

Question 9:
What is the acceleration produced in a freely falling body of mass 10 kg ? (Neglect air resistance)
Solution :
Acceleration produced in a freely falling body, irrespective of its mass, is 9.8m/s²

Question 10:
When an object is dropped from a height, it accelerates and falls down. Name the force which accelerates the object.
Solution :
Gravitational force of the earth.

Question 11:
Give the formula for the gravitational force F between two bodies of masses M and m kept at a distance d from each other.
Solution :
The gravitational force F between two bodies of masses M and m kept at a distance d from each other is :

Here, Gravitational Constant, G= 6.7x 10^-11Nm²kg^-2

Question 12:
What force is responsible for the earth revolving round the sun ?
Solution :
Gravitational force is responsible for the earth revolving round the sun.

Question 13:
What name has been given to the force with which two objects lying apart attract each other ?
Solution :
Gravitational force causes two objects lying apart attract each other.

Question 14:
What type of force is involved in the formation of tides in the sea ?
Solution :
Gravitational force (exerted mainly by the moon and to some extent by the sun) is involved in the formation of tides in the sea.

Question 15:
Which force is responsible for holding the solar system together ?
Solution :
Gravitational force of the sun holds the solar system together.

Question 16:
What is the weight of a 1 kilogram mass on the earth ? (g = 9.8 m/s²).
Solution :
Weight, W = m x g
= 1 kg x 9.8m/s²=9.8 N

Question 17:
On what factor/factors does the weight of a body depend ?
Solution :
The weight of a body is directly proportional to its mass. It also depends on the acceleration due to gravity which varies from place to place.

Question 18:
As the altitude of a body increases, do the weight and mass both vary ?
Solution :
Weight of the body varies with altitude; mass of an object is constant.

Question 19:
If the same body is taken to places having different gravitational field strength, then what will vary: its weight or mass ?
Solution :
Its weight varies; mass of an object is constant.

Question 20:
If the mass of an object be 10 kg, what is its weight ? (g = 9.8 m/s²).
Solution :
Weight, W = m x g = 10 x 9.8 =98 N

Question 21:
The weight of a body is 50 N. What is its mass ? (g = 9.8 m/s²).
Solution :
Weight, W = m x g

Question 22:
A body has a weight of 10 kg on the surface of earth. What will be its weight when taken to the centre of the earth ?
Solution :
Its weight will be zero as value of g is zero at the centre of the earth.

Question 23:
Write down the weight of a 50 kg mass on the earth, (g = 9.8 m/s²).
Solution :
Weight, W= m x g =50 x 9.8=490N

Question 24:
If the weight of a body on the earth is 6 N, what will it be on the moon ?
Solution :
Weight of the body on the surface of moon will be 1N approx. as the value of g on the surface of moon is one-sixth that of the earth

Question 25:
State whether the following statements are true or false :
(a) A falling stone also attracts the earth.
(b) The force of gravitation between two objects depends on the nature of medium between them.
(c) The value of G on the moon is about one-sixth  of the value of G on the earth.(d) The acceleration due to gravity acting on a freely falling body is directly proportional to the mass of the body.(e) Tire weight of an object on the earth is about one-sixth of its weight on the moon.

Solution :
(a)True
(b)False
(c)False
(d)False
(e) False